1. Field of the Invention
The present invention relates to a method and a device for series compensating a rotating electric alternating current machine connected, directly or via a static current converter, to a three-phase distribution or transmission network, wherein the stator winding of the alternating current machine is Y-connected. The invention also relates to a rotating electric machine provided with such a series compensation device.
The invention refers primarily to electric alternating current machines intended for use as generators in power stations for generating electric power. A typical operating range may be 36 to 800 kV, so that they can be connected directly to all types of high-voltage power networks. This is possible thanks to the use of high-voltage insulated electric conductors with solid insulation similar to cables for transmitting electric power, in the following termed high-voltage cable. The cable is also provided with an outer semiconducting layer with the help of which its potential in relation to the surroundings is defined.
2. Discussion of the Background
A conductor is known through U.S Pat. No. 5,036,165, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glassfiber. It is also known to provide conductors in a dynamo-electric machine with such an insulation, as described in U.S Pat. No. 5,066,881 for instance, where a semiconducting pyrolized glassfiber layer is in contact with the two parallel rods forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glassfiber. The pyrolized glassfiber material is described as suitable since it retains its resistivity even after the impregnation treatment.
Series compensation on both high voltage transmission networks and distribution networks is already known.
Series compensation on both high voltage transmission networks and distribution networks is already known. It is also well known that geomagnetically induced currents can cause harmful heating in directly grounded power network system
From U.S Pat. No. A1, 4,341,989 a device is also previously known for phase compensation of a multiphase rotating electric alternating current machine by connecting in series or in parallel with each phase winding a capacitive element on the upside of the winding.
The object of the present invention is to provide a new method and a new device for lowering the system reactance through series compensation of the alternating current machine in question as well as for preventing geomagnetically induced currents.
This object is achieved with a method and a device of the type described in the introduction, having the features defined herein.
According to the invention, thus, the compensation is performed on the down side of the windings, so that low-voltage insulated capacitors can be used, which is not possible with series compensation in high-voltage transmission networks according to known technology. Less expensive capacitors can therefore be used in the device according to the invention, since they are protected by the machine itself and connected to the neutral point, which is at low potential in relation to earth. This solution is especially advantageous for the type of machines to which the present invention relates, since their upside is intended to be connected directly to high-voltage power networks.
According to an advantageous embodiment of the device according to invention, an over-voltage protection means is connected in parallel with the capacitors so that they are protected from any over-voltages that may appear in the event of a fault condition.
According to a second advantageous embodiment of the device according to the invention, a bandstop filter is arranged between the Y-point of the capacitor bank formed by the capacitors and the earth point of the distribution or transmission network, possibly with a low-ohmic resistor connected between the bandstop filter and the earth point. This resistor may be a neutral-point resistor, dimensioned for a harmless earth-fault current of a few tens of amperes. An earth fault in the alternating current machine or the generator is able to emit an earth-fault current via this resistor but, by controlling the earth-fault current, measures can be taken to disconnect the generator or possibly the faulty phase.
From a power network point of view any increase in transient machine reactance can also be efficiently compensated in this manner.
In the machine according to the invention the windings are preferably composed of cables having solid, extruded insulation, of a type now used for power distribution, such as XLPE-cables or cables with EPR-insulation. Such cables are flexible, which is an important property in this context since the technology for the machine according to the invention is based primarily on winding systems in which the winding is formed from cable which is bent during assembly. The flexibility of a XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable 30 mm in diameter, and a radius of curvature of approximately 65 cm for a cable 80 mm in diameter. In the present application the term xe2x80x9cflexiblexe2x80x9d is used to indicate that the winding is flexible down to a radius of curvature of the order of four times the cable diameter, preferably eight to twelve times the cable diameter.
Windings in the present invention are constructed to retain their properties even when they are bent and when they are subjected to thermal stress during operation. It is vital that the layers retain their adhesion to each other in this context. The material properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. In a XLPE-cable, for instance, the insulating layer consists of cross-linked, lowdensity polyethylene, and the semiconducting layers consist of polyethylene with soot and metal particles mixed in. Changes in volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers in relation to the elasticity of these materials, the radial expansion can take place without the adhesion between the layers being lost.
The material combinations stated above should be considered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range of 10xe2x88x921-106 ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the scope of the invention.
The insulating layer may consist, for example, of a solid thermoplastic material such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polymethyl pentene (PMP), cross-linked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.
The inner and outer semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in.
The mechanical properties of these materials, particularly their coefficients of. thermal expansion, are affected relatively little by whether soot or metal powder is mixed in or notxe2x80x94at least in the proportions required to achieve the conductivity necessary according to the invention. The insulating layer and the semiconducting layers thus have substantially the same coefficients of thermal expansion.
Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl graft polyethylene, ethylene-butyl-acrylate-copolymers and ethylene-ethyl-acrylate copolymers may also constitute suitable polymers for the semiconducting layers.
Even when different types of material are used as base in the various layers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with combination of the materials listed above.
The materials listed above have relatively good elasticity, with an E-modulus of E less than 500 MPa, preferably  less than 200 MPa. The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbed in the radial direction of the elasticity so that no cracks or other damage appear and so that the layers are not released from each other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as the weakest of the materials.
The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconducting layer is sufficiently large to contain the electrical field in the cable, but sufficiently small not to give rise to significant losses due to currents induced in the longitudinal direction of the layer.
Thus, each of the two semiconducting layers essentially constitutes one equipotential surface, and these layers will substantially enclose the electrical field between them.
There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulating layer.